MC240 - Back to Life
- Thread starter chrisinsc
- Start date
RShackleford
Active Member
This is the variation that I describe (for the MC40) in the post I linked earlier:
http://www.audiokarma.org/forums/showpost.php?p=7348757&postcount=2
The key difference is whether (or not, in the older units) a single power supply output goes to those resistors and to the center tap on the primary of the output transformers. You need to figure out which version you have, because the variation in the power supply means that R17, R18, R40, and R41 would have different values (30K and 27K, instead of 56K and 51K); depending upon whether those resistors are attached to the same 435vdc as the transformers, or to the separate 300vdc supply.
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chrisinsc
Active Member
I sent you a private message to seek clarification on this.
sxty8goats
Super Member
It took me a bit to locate it. The biggest confirmation is that it is a 5%(gold) resistor and the other 56Kohm resistors are 10%(silver) It is one of the resistors that I reused as it tested in spec and originally wasn't in Jim's kit.
I'm 99% certian that you are correct on R60, Br-Blk-Y-Gold (100K 5%) running between top row 20 and middle row 20.
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chrisinsc
Active Member
I detached one end of each existing resistor and the 100K measured 108K and the 56K measured 60.2K. I went ahead and just left the old ones with one end attached and put the new ones which measure 99.5K and 56.2K in place temporarily. I also have been experimenting with different values in series with SR3. However, I think some of the caps in the audio path are so bad I can't really measure voltages at tube pins until I have replaced them. Voltages are all over the place with one pin 8 on a power tube reading zero volts.
RShackleford
Active Member
I am posting about the MC40, which is simply the mono version of your MC240. There are two variations in the power supply. The older type provides 400+ vdc to the center tap on the primary side of the output transformers, and a lower 300+ voltage to R17,18,40,41; the newer version supplies the same 400+ voltage to those resistors and adjusts the values accordingly. The power supplies for the MC40 and the MC240 are virtually identical, and same variation occurs, so I thought the MC40 stuff would be relevant to you.
I am concerned you could end up with some hybrid, particularly where the R17,18,40,41 values are wrong for the PS configuration you have.
Unfortunately, most schematics you see (for either amp) show one or the other variant, not both. So you could end up with the wrong schematic. The older version is shown here:
http://www.tubebooks.org/file_downloads/McIntosh/MC240_sch.pdf
The newer schematic is shown in post #56 (above), and also shows the dotted lines which try to show the variants, but without the text for the notes. I tried to attach the complete manual for my MC40, which shows the whole thing, and I think it will make sense. (The forum would not let me attach it here, as I earlier attached it to another thread, so you should find it here:
http://www.audiokarma.org/forums/showpost.php?p=7238352&postcount=10
I actually converted one of my MC40s from the old to the new, so they'd be the same. The new is also simpler, as it has one fewer power-supply output, so fewer individual capacitors required (IOW, a single mondo cap would work for C25A/C25B/C26A, with a C26B being something much smaller, an axial lead part should suffice as the capacitance and voltage are much lower).
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chrisinsc
Active Member
Thanks for the pm responses. I now understand this. Very helpful.
chrisinsc
Active Member
And so it goes that the amp I have has resistors for running the 400 plus vs the 300v to r17, 18, 40, and 41. First, I have to say that trying to retain the old components has completely lost any appeal. I detached r17 and 18 and they are 10% plus out of spec just like r60 and r61 were. Secondly, I feel silly for not ordering the resistor kit along with the caps!
I don't recommend that you try and reuse or save the original resistors. You'll never know if they are all within spec after they've warmed and cooled several cycles after you've completed the rebuild. Tracking down the problem after will be a real pain.
If you want to remain faithful to the original design/sound you can purchase new cc units if you wish. You'll at least know they are within spec.
My two cents FWIW
dshoaf
That high voltage buzz
chrisinsc
Active Member
Well thanks for excellent help and encouragement. This is a wonderful place and you all make learning this stuff fun and respectful as we new folks stumble around a bit. I appreciate it.
So, I guess I will re-wire the filter cap section and replace the caps that are coming today and then go through the resistors. I'm getting very good at desoldering the ends of the resistors and lifting them up.
This amp has many hours of running in the last weeks since I got it so I would assume the existing resistors are what they are when I measure them.
So, I guess I will re-wire the filter cap section and replace the caps that are coming today and then go through the resistors. I'm getting very good at desoldering the ends of the resistors and lifting them up.
This amp has many hours of running in the last weeks since I got it so I would assume the existing resistors are what they are when I measure them.
RShackleford
Active Member
Well, it was worth checking. Plus, to be aware that the schematic you had might not have been the right version (sounds to me like the tubebooks one I linked earlier IS the right one - not the one in post #56).
Some might think it silly, but I'd consider converting to the new PS. It's just simpler. Only change in the circuit board, I believe, is that R17,18,40,41 are 56K and 51K (instead of 30K and 27K). But it made a lot more sense for me, since I have two amps (since MC40 is mono version of MC240), so good to have them the same.
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chrisinsc
Active Member
Yes, the filter caps as originally wired and r17 and r18 testing above 50K indicate that I do indeed have the circuit you posted. In fact, when I put the new 150K resistor between the A sides of c25/c26 and the B sides I get 172v on the B side when the A side is 435v. The original resistor there measures 160K. I am trying that one today and also sourcing a range of various resistors to see about getting the correct voltages at the various locations exiting the power section.
RShackleford
Active Member
Not what I posted (unless you mean the MC40 version of it), but rather the one in post#56 of this thread, right ? So never mind what I said in my last post, you DO have the newer version of the circuit. That is nice, because is the future (and you WILL likely have to replace the PS electrolytics again at some point), you can just use a single 200+ uF 500+V cap for C25A/C26A, and a single much smaller (voltage and capacitance) and easier to source part for C25B/C26B.
chrisinsc
Active Member
So almost done with the recap. Just a few more tonight. I am working from schematic 154-319 now and am curious about couple things.
- This design has pin 1 on V2 and V5 at 275v and the others have this at 205v? I am measuring around 250v.
- When measuring 435 at the power section I also get 435 at pins 3/4 on the power tubes instead of 430.
- Is it a problem measuring up to .9v at pin 8 on the power tubes? I have between .7 and .9 on the four tubes.
- This design has pin 1 on V2 and V5 at 275v and the others have this at 205v? I am measuring around 250v.
- When measuring 435 at the power section I also get 435 at pins 3/4 on the power tubes instead of 430.
- Is it a problem measuring up to .9v at pin 8 on the power tubes? I have between .7 and .9 on the four tubes.
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chrisinsc
Active Member
On another note I had a bad ax7 and that was goofing up the voltages on pin 8 of two of the power tubes. I must say I would never start a project like this again without getting a set of known good tubes. I cleaned and massaged the sockets but a bad old tube is a bad old tube.
And still another note, between mc40 and mc240 schematics there is quite a bit of variation in voltages over time. Definitely a change took place in both models with respect to the au7 voltages and the power tubes.
And still another note, between mc40 and mc240 schematics there is quite a bit of variation in voltages over time. Definitely a change took place in both models with respect to the au7 voltages and the power tubes.
chrisinsc
Active Member
Been running the amp now for many hours and its sounding pretty good and not getting too hot either at the tubes or the transformers. Left all the original resistors in but measure them all while replacing the caps. The bass slam is definitely happening which is really cool. Really enjoying it. Will run it for a few weeks and then check voltages and resistances again.
sxty8goats
Super Member
I've been thinking of pulling mine out and checking voltages too. Along with running a set of KT66 tubes. I've heard a lot of good about the Gold Lion tubes but the set of 4 would draw an additional amp total from the power transformer. I'm not sure if it has the head room for that. Other than that they are 6L6's with a bit more head room.
chrisinsc
Active Member
I have a set of chinese KT66's in my fender twin which I re-biased this weekend. They sound great in that guitar amp but they are running pretty hot and I point a fan at them when I play.
sxty8goats
Super Member
I have a Vibro Champ that I'm thinking about putting on the bench. She is an early 70's model I think. I had her gone over in the early 90's and now she sports some truly budget tubes. I didn't know any better and the amp tech did what he did. It went in because it was spotty on volume. It would be solid and loud for 10 minutes and then cut out to 2/3 volume with no life. That bit was fixed. And she sounds good. But I want to know what is in there.
